TRACE is a solar mission of NASA/GSFC within the SMEX (Small Explorer) program. The objective is to study the three-dimensional magnetic structures of the sun - which emerge through the visible surface of the sun, the photosphere, and to define both the geometry and dynamics of the upper solar atmosphere - the transition region and corona. TRACE is the name of the prime instrument (as well as of the mission), which is mounted onto a SMEX spacecraft designed and built by GSFC. TRACE is the first US solar research satellite since the Solar Maximum Mission (launch of SMM Feb. 14, 1980). 1)2)3)4)

TRACE explores the magnetic field in the solar atmosphere by studying:

• the 3-dimensional field structure

• its temporal evolution in response to photospheric flows

• the time-dependent coronal fine structure

• the coronal and transition region thermal topology.

Figure 1: Photo of the TRACE spacecraft in the cleanroom during assembly (image credit: NASA)

Spacecraft:

TRACE is a single-instrument sun-pointing spacecraft in LEO; it was manufactured by NASA/GSFC. The spacecraft is three-axis-stabilized using the TRACE telescope as a fine sun sensor. A S/C pointing accuracy of 20 arcseconds is required. The ACS (Attitude Control System) is using one digital sun sensor, six coarse sun sensors, one three-axis magnetometer, three two-axis inertial gyros, as well as the TRACE instrument, to sense attitude (the latter two instruments for fine pointing to < 5 arcsec). Actuation is provided by three magnetic-torquer coils and four reaction wheels,. The ACS uses the S/C computer to perform closed-loop attitude determination and control.

Figure 2: Illustration of the TRACE spacecraft (image credit: LMSAL)

The command and data handling subsystem uses a 32-bit on-board processor (80386/ 80387) with 300 MByte solid-state memory for science data. A MIL-STD-1553 data bus is used to connect all subsystems and instruments. Science data is passed over the RS-422 interface at rates up to 900 kbit/s.

Electric power of 220 W is provided by GaAs solar cells, deployed in four panels with a total area of 2 m2. A 28 V unregulated bus distributes the power, a 9 Ah super NiCd battery provides energy storage. The mission design life is 1 year. S/C mass = 250 kg.

Launch: The TRACE spacecraft was air-launched on April 2, 1998 (UT) on a Pegasus-XL vehicle; the aircraft (L-1011) took off from VAFB and the Pegasus-XL payload was released off the central California coastline.

• NASA retired the TRACE mission in June 2010. The last observations of the sun were conducted on June 21, 2010. During its 12 year mission, TRACE produced millions of stunning images and contributed to more than 1,000 scientific publications. 5)

TRACE provided images at five times the magnification of those taken by the EIT (Extreme UV Imaging Telescope) aboard the SOHO (Solar and Heliospheric Observatory) mission. Many details of the fine structure of the corona were observed for the first time. Early in its mission, it discovered the fine-scale magnetic features where enhanced heating occurs at the footpoints of coronal loop systems in solar active regions, which later became known as "coronal moss." 6)
Note: The coronal moss is the bright, reticulated pattern observed in many EUV images of solar active regions. These regions are the footpoints of high-temperature active region loops, and are a potentially rich source of information on the conditions in high-temperature coronal loops.

Legend: The image of Figure 5 was taken by TRACE on a quiet day in September 2000. It shows that even during ”off days” the sun's surface is a busy place. Shown in ultraviolet light, the relatively cool dark regions have temperatures of thousands of degrees. 8)

• The TRACE spacecraft and its payload are operating nominally in March 2010. The project expects to continue the mission until April/May 2010. TRACE's future beyond that remains unknown, but NASA does not now have intentions to continue it as a science mission. 9)10)

• The TRACE spacecraft and its payload are operational in 2009 (> 11 years after launch, exceeding the design life of 1 year by far). TRACE has collected several million images of various solar features in coordination with SOHO as well as with other solar observatories around the world. 11)

• On April 2, 2008, the TRACE mission was 10 years on orbit.

• As of 2007, plans are to keep TRACE operational until 2009 to enable cross-calibration of the instrument with NASA's SDO (Solar Dynamics Observatory) mission (launch on February 11, 2010), to ensure continued access to high-resolution observations of the EUV corona by the community. 12)

• TRACE has exceeded all requirements and expectations. The findings have been substantial, therefore the mission has been repeatedly extended. So far, there has been no measurable degradation of the sensor, mechanisms, optics, or electronics.

Legend to Figure 6: Extending above the photosphere or visible surface of the sun, the faint, tenuous solar corona can't be easily seen from Earth, but it is measured to be hundreds of times hotter than the photosphere itself. What makes the solar corona so hot? Astronomers have long sought the source of the corona's heat in magnetic fields which loft monstrous loops of solar plasma above the photosphere. Still, new and dramatically detailed observations of coronal loops from the orbiting TRACE satellite are now pointing more closely to the unidentified energy source. Recorded in extreme ultraviolet light, this and other TRACE images indicate that most of the heating occurs low in the corona, near the bases of the loops as they emerge from and return to the solar surface. The new results confound the conventional theory which relies on heating the loops uniformly. This tantalizing TRACE image shows clusters of the majestic, hot coronal loops which span 30 or more times the diameter of planet Earth. 13)

• The extended mission of TRACE started in April 1999 (since the mission life was initially set to 1 year).

Sensor complement: (TRACE)

TRACE (Transition Region and Coronal Explorer):

The instrument was designed and built by a consortium led by LMSAL (Lockheed Martin Solar and Astrophysics Laboratory) of Palo Alto, CA, SAO (Smithsonian Astrophysical Observatory) of Harvard University, and GSFC. The TRACE science team is from diverse institutions: LMSAL, GSFC, SAO, MSU (Montana State University), Stanford University, and the University of Chicago. The objective is to collect comprehensive multispectral images of solar plasmas at temperatures from 104 - 107 K with 1 arcsecond spatial resolutions and excellent temporal resolution and continuity. An overall science goal of TRACE is to explore the relation between diffusion of the surface magnetic fields and changes in heating and structure throughout the transition region and corona (to understand how energy is transported from the solar surface into the outer atmosphere). 14)

Each quadrant of the primary mirror is coated for sensitivity to a different wavelength range. Light entering the instrument passes first through the entrance filter assembly which transmits only far and extreme UV. Visible and near UV radiation (hence, most of the solar energy) are reflected back into space. Radiation transmitted through the entrance filters passes to a shutter that blocks three quadrants of the aperture, so that only one quadrant of the telescope is illuminated at a time. Photons passing the shutter's open quadrant proceed to the primary mirror, encountering a multilayer coating for a narrow-band EUV quadrant, or a broadband coating for the UV quadrant. The segmented coatings on solid mirrors form identically sized and perfectly coaligned images.

The reflected beam from the primary mirror proceeds to the secondary mirror which reflects it towards the focal plane. The secondary mirror is active to correct for pointing jitter and has coatings matching those on the four quadrants of the primary mirror. - Further stations in the radiation path are the filter wheels and the focal plane shutter. The final element in the optical train is the CCD camera. The on-board computer permits very flexible use of the CCD array, including adaptive target selection, data compression, and fast operation for a limited FOV. TRACE is of SXT (Soft X-ray Telescope) heritage of Solar-A (Yohkoh).

The GT (Guide Telescope) and ISS (Image Stabilization System) perform two primary functions. In a cooperative mode with the spacecraft ACS, The GT directs pointing maneuvers to the desired solar targets. The ISS provides jitter removal to < 0.1 arcsec rms based on error signals from the GT.

The GT has a 2.5 cm aperture and an effective focal length of 187 cm. It forms a filtered (5600 Å with a FWHM of 500 Å) image of the sun which falls onto a limb sensor unit that consists of four pairs of photo diodes mounted at 90º intervals. The intensity difference between opposite diodes is a linear displacement error signal with a range of ±120 arcseconds. Signals are sent from the instrument to the spacecraft ACS which adjusts the pointing to keep the limb sensor centered. The ACS is continually adjusting the pointing of the spacecraft to keep the limb sensor centered, which results in the main telescope FOV moving to the desired offset location. Moving from one position on the disk to another typically takes about 30 s and is < 2 minutes in the most extreme case.

Figure 8: The pointing system of the TRACE instrument (image credit: LMSAL)

Observations: The TRACE instrument FOV observes about 1/10th of the solar disk at a time. TRACE is best suited for continually monitoring within this FOV at a high temporal cadence. Higher time cadence is obtained by only taking a subset of the available wavelengths; often one of the EUV wavelengths, on UV wavelength and the WL channel for alignment with other data. A sequence of this sort can achieve a temporal cadence of about 20-30 seconds. Such observations are then constrained by the available mass memory and telemetry. Spacecraft memory use is optimized by onboard 12 bit JPEG compression of data, with the level of compression being dependent upon wavelength and required image quality.

The time frame of the TRACE mission has the advantage of operating simultaneously with the SOHO (ESA/NASA, November launch 1995) mission. This implies coordination of observations from both S/C and the merging of datasets collected from TRACE and SOHO.

TRACE instrument computer: The instrument computer system consists of 2 processors: the control computer (CC) and the Data Handling Computer (DHC). The CC is an i86 architecture machine with a radiation hardened 80C86 processor and additional electronics, some standard and some custom. The DHC is a custom image processor based on the AMD 2910 bit-slice architecture. The CC controls electromechanical components, manages the spacecraft command and housekeeping interfaces, and maintains the cadence of all operations including science observing sequences. The DHC receives raw data from the camera system, processes it, and outputs formatted data to the spacecraft's bulk memory via a high speed serial interface. Based on information supplied by the CC, the DHC performs various operations on the image data including flare detection, exposure evaluation, time averaging, spatial averaging, sub-field extraction, and JPEG compression.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.